Thermoplastic-photoconductor holographic recording medium has generally been in the form of several transparent layers over a transparent substrate. Thus a substrate such as nesa glass or a flexible substrate such as Mylar is first coated thereon an optically transparent electrically conductive layer, then a photoconductor layer, and finally a thermoplastic layer.
In prior U.S. Pat. No. 4,131,462, assigned to the same assignee as the present invention, an optically transparent electrically conductive layer is described as a thin film of metal such as gold or indium-tin oxide, the photoconductive layer is described as polyvinyl carbazole (PVK) doped with trinitrofluorenone (TNF) and the top layer is described as a thermoplastic such as resin or synthetic polymers. The transparent conductor layer beneath the thermoplastic-photoconductor medium can also be a number of other materials such as platinum, aluminum, and nickel-chromium (NiCr). NiCr used as the conductor layer has advantages both of low cost and of low reflectivity at the PVK-NiCr interface. This low reflectivity of the NiCr is beneficial to hologram recording and the lower cost is attractive. NiCr presents problems, however, in that when electrostatic charges are applied across the capacitor formed by the free surface and the NiCr electrode, electrons tend to be injected into the PVK causing poor charge acceptance and therefore resulting in poor or no hologram formation. Ideally, the interface between the photoconductor layer and the conductive layer should be a perfect blocking contact (no dark charge injection from NiCr into the photoconductive layer) so that a desirable field ratio (close to 1) between the photoconductor and thermoplastic layers can be maintained. Thus, the charge contrast created during exposure can be retained, resulting in high diffraction efficiency holograms. In reality, however, the NiCr conductive layer departs from the desirable characteristics in that there tends to be an injection of charge into the photoconductive layer. Measurements show the photoconductor layer loses the electric field as the device is charging which results in loss of the field and in loss of charge contrast during and after it is exposed. Thus, the resulting diffraction efficiency of the hologram is weak or none at all. That is to say the NiCr-PVK interface forms a poor blocking contact. This is a problem which does not exist when the more expensive gold is used as the conduction layer.
The present invention is directed to treating the NiCr surface with chemically active species of oxygen such as free radical oxygen, ionized oxygen or atomic state oxygen. This can be carried out in an oxygen plasma environment, for example. In so treating the NiCr surface a mono atom thick layer of the surface of NiCr becomes an oxide layer which is an effective charge blocking layer. After the treatment, the thermoplastic-photoconductor device shows improved performance in terms of cycle-to-cycle repeatability and diffraction efficiency. The problem of charge leakage at the PVK-electrode interface is not unique to NiCr. It occurs also to a lesser extent with indium-tin oxide and it can be corrected by the same technique.